1. Field of the Invention
This invention relates to clad aluminum material, such as brazing alloy sheets of the type comprising a core of structural material and having on at least one side a cladding of a brazing alloy, and more particularly to a method by which slots are formed in sheets whereby the cladding remains on the sidewalls of the slot.
2. Description of the Prior Art
Brazing alloy sheets typically comprise an aluminum-based core with at least one side having a cladding of a cladding material. The cladding material is typically an aluminum-based brazing alloy containing silicon as the main alloying ingredient. The brazing alloy has a lower fusing point than the core material, and is used to facilitate brazing of a finished assembly made using the sheets. For example, the aluminum core may be 3000 series aluminum composition metal having a thickness of about 0.3 mm to 0.35 mm and a fusing point of about 350xc2x0 C., while the brazing alloy may be a 4000 series aluminum composition metal having a fusing point of about 300xc2x0 C.
Brazing sheets of this sort are commonly used in the manufacture of heat exchangers. In the typical construction of such heat exchangers, the heat exchanger has two generally parallel manifolds with a plurality of cooling tubes connected between them, and plurality of cooling fins attached to the tubes. The tubes are through the tubes to the other manifold. Heat exchangers of this type are used in automotive vehicles for several purposes, such as a radiator for cooling the engine and as charge air coolers.
The manifolds may be made of a molded plastic tank having an aluminum header plate or cover in which the plurality of spaced slots is formed for connection to the cooling tubes. Alternatively, each of the manifolds may be formed completely from aluminum and have a plurality of spaced slots along one side. The manifolds are also provided with connections for the introduction of the cooling medium. The header plates are usually made from a brazing sheet. The cooling tubes may also be made from abrazing sheet, or more commonly, are made from a nonclad aluminum sheet which is rolled into a tube and welded along the longitudinal seam. After assembly of the aluminum components of the heat exchanger, the assembly is brazed in an oven or furnace to hold the elements together as assembled and provide a sealed vessel for the cooling medium.
For purposes of making heat exchangers, the brazing sheets must exhibit a number of important properties, including corrosion resistance, formability, post-braze strength, and brazability. The brazabilty of the material is especially important since the finished heat exchangers rely upon the brazing operation in order to assure a fluid tight connection between the components, especially between the slots and the tubes. If a leak occurs, the cooling medium can quickly leak out of the heat exchanger creating a potentially dangerous condition.
The slots for the tubes have been commonly formed in the manifold or header plate in a piercing operation in which piercing members strike a blank formed from the brazing sheet and punch out the material to form the slots. The piercing operation removes the cladding material, along with the removed material, so that thereafter when a brazing operation is performed, the brazing material may not readily flow into the slot around the tube, and the connection can be inherently weak and may be one of the most likely portions of a heat exchanger to develop a leak.
The brazing of the completed assembly helps to prevent leaks, but the effectiveness of the brazing operation is dependent upon the cladding layer on the aluminum brazing sheet. When the slot is simply punched out, there is no cladding material on the side walls of the slot, and the brazing operation will not necessarily create a fluid tight connection between the slot and the tube. This is particularly likely where the tubes are formed from a nonclad aluminum sheet.
Various attempts have been made to improve the connection between the tubes and the headers. For example, in U.S. Pat. No. 4,577,380 issued to Warner, a coating of polytetrafluoroethylene (PFTE) is placed on the material sheet. A heated tool having a forward rounded nose is then used to make holes in the PTFE layer and push the material through the holes in the material sheet so that the PTFE lines the hole. This patent, however, does not recognize any need to deal with the brazing layer of a brazing sheet. Furthermore, the technique used for the PTFE layer would not be suitable for manipulating the brazing layer of a brazing sheet since the brazing layer is fixedly clad on the base layer of the sheet.
U.S. Pat. No. 4,150,556, issued to Melnyk, discloses a method for forming ferrules around the openings in the header used to mount the tubes. The method is a relatively complicated three step process which involves forming a pair of small holes at each end of the desired opening, and splitting the bottom wall of a depression along the length of the opening to form the ferrules. The method disclosed in this patent makes no effort to deal with clad surfaces, and there is nothing to indicate that the cladding would be present along the inside of the ferrules or would be present on the inside of slots if the ferrules were not formed by this method.
U.S. Pat. No. 5,228,512, issued to Bretl, discloses another method for forming slots in the header in which, first, initial apertures are first formed in the header which are smaller than the desired finished slots, and then the header is punched to enlarge the apertures into the desired slots and form flanges around the slot. This method relies upon the remaining material around the preliminary apertures to supply the material for the flanges. It has limitations regarding the thickness of the material that can be used, and due to the forming process it can result in fatigue problems and cracking at the ends of the slots.
The present invention provides an improved method for forming slots in clad aluminum materials, such as brazing sheets, in which the cladding is maintained on the side walls of the slot, so that when the brazing operation is performed, a fluid tight seal is formed between the slots and the tubes inserted therein, even if the tubes are made from nonclad material.
When the ends of the cooling tubes are assembled in the slots, the cladding on the side walls of each slot comes into contact with the cladding on the exterior of the tube. During the subsequent brazing operation, the brazing material layers on the tube exterior and the slot interior together facilitate the brazing of the tubes in the slots, increasing the likelihood of a fluid-tight connection between the tubes and the slots.
In accordance with the present invention, the slots are formed in a progressive die operation, which comprises a pre-extrusion station in which the brazing sheet is pre-formed or extruded into a plurality of depressions having dimensions greater than that of the desired slots, forming clad sidewalls, and a piercing station in which the entire bottom of the depression is removed and a slot is formed while the clad sidewalls are maintained. This two-step forming operation provides a method by which the brazing sheet is deformed in order to provide the cladding layer around the inside of the finished slots. With the presence of the cladding layer on the inside sidewalls of the slot, the effectiveness of the brazing operation is increased.
The method of the present invention has been found to be useful in forming slots in relative thick aluminum sheets. The method avoids the fatigue problems of the prior art, and reduces the cracking which may ordinarily appear at the ends of the slots.
The slot forming method of the present invention can be achieved with a minimal increase in expense. If a progressive die operation has been previously used to form the headers, the method of the present invention can be incorporated into the progressive die operation by adding two additional stations, replacing the single step piercing operation used to form the tube slots in the header with a two-step pre-extrusion and piercing operation. The resulting forming operation will provide slots on the brazing sheet with the cladding layer of brazing material present on the inside walls of the slot, so that the subsequent brazing operation used to fix the tubes in the slots will be stronger and more fluid tight.
These and other advantages are provided by the present invention of a method of forming slots in a sheet of material having a cladding layer on at least one surface, comprising the steps of pre-forming the material by forming depressions in the sheet, each of the depressions having a bottom wall having the approximate dimensions of a desired slot, each of the depressions also having sidewalls with the cladding thereon, the sidewalls extending from the bottom wall to the sheet which has not been pre-formed; and then removing the entire bottom wall of the depressions in a piercing operation, forming a slot in the sheet where the bottom wall was, and leaving the slot with substantially clad sidewalls.